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Science Report Describes Deepwater Horizon Oil Plume as of June

In mid-June, two months after the 20 April blowout of BP’s Deepwater Horizon drilling rig, the resulting oil plume had reached huge proportions, peer-reviewed research published 19 August on the Science Express Web site suggests.

The research also shows that levels of dissolved oxygen within the plume had not dropped enough by mid-June to suggest bacteria were breaking down the oil, according to Richard Camilli of the Woods Hole Oceanographic Institution (WHOI), who directed the study.

Between 19 and 28 June, Camilli and his colleagues concluded, the plume measured approximately 22 miles long and 1.2 miles wide. It had by then reached a height of more than 650 feet and depths in excess of 3,600 feet below the Gulf of Mexico, the researchers reported in their Science Express paper.

At a press conference at the National Press Club, Camilli said: “The data suggest that this plume extended for much further than we tracked it,” or longer than 22 miles (35 kilometers). But he and other Science authors emphasized that their research is only a “forensic snapshot,” based on deep-sea conditions as of mid-June. The plume seemed at that time to be following the continental shelf along the sea floor, Camilli added.

After receiving a fast-turnaround grant called a “RAPID” award from the U.S. National Science Foundation (NSF), Camilli’s team sent the automated underwater vehicle Sentry on a 10-day reconnaissance of the plume that erupted from the Macondo Prospect oil field following the Deepwater Horizon disaster. The NSF-supported Sentry vehicle zigzagged through the plume, recording information about the water’s chemical composition and biological activity at various depths.

Richard Camilli and Christopher Reddy

The researchers took deep-water measurements using a set of instruments cabled to the ship, Endeavor, another NSF-funded vessel operated by the University of Rhode Island. The Science authors also received “outstanding” assistance from the National Oceanic and Atmospheric Administration (NOAA) as well as BP and the U.S. Environmental Protection Agency, said team member Christopher Reddy, a marine geochemist and oil spill expert with the WHOI.

Dissolved oxygen levels were measured in three ways: microelectrode sensors, “Winkler titration tests,” and mass spectrometry. Being able to triple-check the team’s dissolved-oxygen measurements was important, Reddy explained, because the microelectrode sensors can become fouled by oil, resulting in erroneous results.

Measuring the spill. Video from the National Press Club news conference, 17 August 2010, on the new research published in Science that assessed the size of the plume from the Gulf oil spill.

According to the WHOI news release, the researchers began tracking the plume three miles from the Deepwater Horizon well head, extending 22 miles, until the approach of Hurricane Alex forced them out of the area. “We’ve shown conclusively not only that a plume existed, but also defined its origin and near-field structure,” said Camilli, chief scientist of the research cruise and lead author of the Science Express paper. “In June, we observed the plume migrating slowly [at about 0.17 miles per hour] southwest of the source of the blowout.”

In addition to measurements of the size of the plume as of mid-June, the researchers reported that about 6% or 7% of all the so-called BTEX hydrocarbons leaked from the well—benzene, toluene, ethylbenzene and xylenes—were in the plume. These volatile hydrocarbons make up a small fraction of all the compounds in the oil but are relatively easy to measure.

Slow burn. To help dissipate the plume, crews in May burned floating oil from the Deepwater Horizon spill in the Gulf of Mexico.
[Public domain image by John Kepsimelis, U.S. Coast Guard]

The public may wonder whether the oil plume, if it continues to persist, poses a significant ongoing threat to the Gulf. In the WHOI news release, Reddy said: “We don’t know how toxic it is, and we don’t know how it formed, or why. But knowing the size, shape, depth, and heading of this plume will be vital for answering many of these questions.”

Nick Wigginton, associate editor with Science, said: “The authors of this paper have taken an innovative approach to identifying and sampling part of a deep hydrocarbon plume, including measuring the critical oxygen levels in the water column. These levels are obviously important biologically but also reveal the extent of natural degradation of the oil by microorganisms.”

The presence of such a plume shows that hydrocarbons can move into deep marine ecosystems, the authors say. They noted that more research will be needed before scientists know the eventual fate of this material and the full extent of the chemical compounds in the plume.

“The location, depth, flow and direction made this spill like no other, and more work needs to be done,” said Ron Munier, a vice president at the WHOI.

The 19 August Science Express paper is entitled “Tracking Hydrocarbon Plume Transport and Biodegradation at Deepwater Horizon.” The research was funded by the U.S. National Science Foundation. Additional authors on the Science Express paper, in addition to Camilli and Reddy, were Dana Yoerger, Benjamin Van Mooy, James Kinsey, Cameron McIntyre, and Sean Sylva of the WHOI; Michael Jakuba of the University of Sydney; and James Maloney of Monitor Instruments Company, LLC.